The present invention relates to monitoring of braking for a vehicle equipped with a regenerative braking means, for example an electric braking means, and a complementary braking means, for example a hydraulic braking means.
The vehicle can, for example, be an electric or hybrid vehicle.
On a vehicle equipped with at least one electric traction or propulsion motor, it is possible, under certain conditions, to use the electric motor as generator and thus obtain an electric braking means.
Such a use is advantageous because, being regenerative, it makes it possible to recover a portion of the energy in order to recharge the batteries.
Nevertheless, in the case of vehicles thus equipped with an electric actuator allowing for regenerative braking, and a hydraulic actuator of a conventional friction brake, it has been observed that, for a given braking set point (i.e. a user set point, that is to say a given pressure on the brake pedal), the level of deceleration is likely to be different according to the distribution between the hydraulic braking and the electric braking.
Referring to FIG. 1, there is represented, over a braking sequence or transition, the trend over time of braking torques and of the acceleration of the vehicle with, on the x axis, the time, bearing in mind that the driver applies a constant braking set point such that the vehicle slows down commensurately, and, on the y axis, the electric torque Ce, the hydraulic torque Ch and the resulting acceleration (or deceleration) γ.
As it happens, the system of distribution between electric braking and hydraulic braking is arranged in such a way as to prevent the electric braking below 7 km/h. The cut-off is gradual, between 14 and 7 km/h. The instant T1 corresponds to a threshold of 14 km/h from which the regenerative braking is gradually deactivated, and the instant T2 corresponds to a threshold of 7 km/h.
The chain-dotted curve represents the torque corresponding to the observed deceleration. As can be seen for the instants slightly before T1 for which the braking is almost entirely electric, the torque applied is greater than the torque applied for the instants slightly after T2, for which the braking is almost entirely hydraulic. This gap is due to the imprecision of the efficiency of the hydraulic actuator.
This can be relatively disturbing for the user, in as much as the user can have the sensation that the vehicle is likely to not use the same braking for a given control.
The electric actuator can have precision of the order of 5%, but the efficiency of the hydraulic actuator can vary with time and exhibit deviations of the order of 30 or 40%. The efficiency of the hydraulic actuator is in fact a function in particular of the state of wear of the brake pads and of the temperature.
There is therefore a need for a response of the vehicles to braking that is more predictable and more constant for the user.
The document US 2008/01291 10 describes a method for monitoring braking with energy recovery comprising:                a step of measuring the acceleration when the vehicle reaches a certain speed for which this acceleration is assumed to result only from the efficiency of the electric braking means;        a step of measuring the acceleration for another speed of the vehicle, this other speed being such that the acceleration is assumed to result only from the efficiency of the hydraulic braking means, these two measurements being taken for one and the same braking set point value applied by the driver,        a step of calculating the ratio of the braking efficiencies, by establishing the ratio of the measured accelerations,        a step of applying the ratio of the braking efficiencies to the hydraulic braking control.        
This method is then repeated until there is convergence to a gap reduced to zero.
The measurements conditions are difficult to obtain, and the convergence is relatively slow. Furthermore, there is a great dependency on the measurement conditions. In particular, the measurement is segmented over speed ranges, which is detrimental to the generality of the method.